It is common practice for fluid samples to be extracted from a pressurized pipeline for “on-line” analysis or laboratory analysis. Such is the case in the natural gas industry wherein the monetary value of the gas is dependent on its composition. The chemical and oil refining industries also have needs for extracting fluid samples from pressurized fluid sources.
Additionally sensors, such as pressure and temperature sensors and corrosion coupons often require insertion into pressurized fluid streams. There are many probe types designed to be inserted into pressurized fluid systems. There are pressure balance insertion methods, such as described in Mayeaux U.S. Pat. Nos. 7,472,615 and 6,701,794, which do not require forcing the probe through a seal. There are smooth walled probe types, such as described in Welker U.S. Pat. Nos. 6,964,517 and 6,827,486, which are forced through a seal into a pressurized fluid by pneumatic or hydraulic means. Another probe insertion method utilizes a threaded male membrane and threaded female nut to force a smooth walled probe through a seal into the pressurized fluid. An example is the Mudiam U.S. Pat. No. 5,106,580.
The aforementioned methods of probe insertion each have drawbacks. For example, the Mayeaux patents require a housing with foot valve which prevents it from being utilized in a horizontal position. The Welker patents describe probes requiring valving and pneumatic or hydraulic cylinders which complicates their construction and operation. The Mudiam patent describes a complex apparatus in which a rod type of probe is inserted by utilizing a separate threaded member.
Another common problem associated with the use of sampling probes, sensors or the like in a high velocity fluid stream (or even lower velocity fluid streams, depending upon the structure of the probe and density of the fluid) relates to the phenomenon of resonant vibration induced in the probe itself due to the flow of fluid therethrough.
According to the manual of Petroleum Measurement Standards (API), Chapter 14, Natural Gas Fluids Measurement, Section 1—Collecting and Handling of Natural Gas Samples for Custody Transfer, (Sixth edition, February 2006) (herein referred to as API 14.1 2006) (the contents of which are incorporated herein by reference thereto):                “API 14.1 2006 Section 7.1:        The design must also consider the possibility of resonant vibration being induced in the probe by high flowing velocities in the pipeline.        API 14.1 2006 Section 7.4.1:        It is industry practice that the collection end of the probe be placed within the approximate center one-third of the pipe cross-section. While it is necessary to avoid the area most likely to contain migrating liquids, the pipe wall, it may be necessary to limit the probe length to ensure that it cannot fail due to the effects of resonant vibration.        Resonant vibration can occur when the vortex shedding frequency resulting from a probe inserted into a flowing fluid is equal to or greater than the probe's natural resonant frequency. Table 1 provides recommended probe lengths for typical diameters based on a maximum natural gas velocity of 100 ft/sec (30.48 m/sec).” (Emphasis Ours).        
Thus, API recognizes the problem of resonant vibration in probes in high flow environments, and its recommendation is to position the probe away from the pipe wall, as well as “limit the probe length to ensure that it cannot fail due to the effects of resonant vibration”, as indicated in the above Section 7.4.1, 1st Para. In fact, Chapter 14 of the above API Standards, Chapter 14, page 14, sets forth recommended maximum probe lengths in an effort to combat resonant vibrations:
TABLE 1Maximum Recommended Probe LengthsProbeRecommended MaxOuter DiameterProbe LengthInches (cm)Inches (cm)0.25 (0.64)2.00 (5.08) 0.375 (0.95)3.25 (8.26)0.50 (1.27) 4.25 (10.80)0.75 (1.91) 6.50 (16.51)“Calculations were based on a maximum recommended probe length Strouhal Number of 0.4, a 0.035 in (0.089 cm) wall thickness, and 316 stainless steel probe construction.”Further, as indicated by the Gas Processors Association “Obtaining Natural Gas Samples for Analysis by Gas Chromatography Standard 2166 (Rev 2005) (hereinafter referenced as GPA 2166-2005, Section 7.5.2:
“It has been an industry practice that the collection end of the probe be placed within the approximate center one-third of the pipe cross-section. While it is necessary to avoid the area most likely to contain migrating liquids (the pipe wall) it may also be necessary to limit the probe length to ensure that it cannot fail due to the effects of resonant vibration.
Resonant vibration can occur when the vortex shedding frequency resulting from a probe inserted into a flowing fluid is equal to or greater than the probe's natural resonant frequency. Table 2 provides maximum probe lengths for typical diameters based on a maximum natural gas velocity of 100 ft/sec.”
TABLE 2Recommended Maximum SampleProbe LengthsRecommendedProbe OuterMaximum ProbeDiameter (inches)Length (inches)*0.2502.000.3753.250.5004.250.7506.50*Calculations were based on a maximum probe length Strouhal Number of 0.4, a 0.035 inch wall thickness, maximum flow velocity of 100 ft. sec. and 316 stainless steel (E = 28,000,000 PSI, r = 7.96 g/cc) probe construction.
Section 7.5.2.1 of the standard specifies specific calculations of the permissible probe length taking into account several criteria including velocity of the fluid, modulus of elasticity of the probe material and its density, and others, and in GPA 2166-2005 Section 7.5.3 it is indicated that:
“Under no circumstances should the Sample Probe be longer than 10”.
Caution: Harmonics may cause embrittlement of the metal. Poorly designed Sample Probes may bend or break off in the flowing gas stream.”
Many other standards also recognize the inherent problem with current probe designs and the harmonics/resonance issue due to the velocity of fluid flow in the testing area, including ISO 10715:1997, EEMUA 138 (Engineering Equipment and Material Users Assoc) (the contents of which are incorporated herein by reference thereto), and provide guidelines as to appropriate probe length and construction depending upon the sample fluid flow, etc).
One reviewing the above standards would quickly discern that the industry accepted practice for combating resonance and vibration issues in the probe is 1) keep the probe in the central area of the pipe and avoid the pipe walls, as well as to 2) limit the probes length and/or beef up its construction to resist such forces, although both guidelines are limiting by nature and as such basically avoiding the problem instead of responding to it.
One would also note there is no noticeable mention of providing a design profile to lessen vortices and associated vortex shedding-induced vibrations, cavitations, or other disturbances in the fluid flow which can lead to vibrations or oscillation-inducing forces, nor is there mention of providing a means of dampening or absorbing any such forces as an alternative to the structural limitation guidelines summarized above.
Put another way, one may criticize the above industry standards for sampling as it may be construed to limit ones ability to take what may be the best sample likely to contain migrating liquids using a probe in a process pipeline, that is, along the inner pipe wall.
Also, the limitation of probe length recommendations summarized above by the standards commissions are their best efforts at preventing failure of the probes due to the effects of resonant vibration. So the probes, to be compliant, must be long enough to extend into the flowing gas stream and be away from the wall. It has been an industry practice to require the probes to be in the center third of the pipeline. However, another requirement is that the probes can not be longer than 10″ in length. Very large diameter pipelines such as 36″ or 42″ pipelines would require probes longer than the maximum recommended length.
The prior art probes such as described in EEMUA 138 (above) include those which incorporate smooth pieces of tubing welded into fittings. When a fluid flows past a cylindrical projection (smooth tube sample probe) in a pipeline, vortices can form at either side of the cylinder. As the fluid velocity and hence Reynolds Number increases, these vortices tend to grow in size, elongate and eventually detach, first from one side of the smooth cylinder and then from the other. As soon as one vortex detaches, another one can be created. This phenomenon is called “vortex shedding” and the frequency at which it occurs is called the “shed frequency”. In developing these vortices the cylinder experiences drag forces in a direction transverse to the fluid flow.
Since the alternate vortices are of opposite signs the cylinder is subject to a periodic transverse force. It has been found experimentally that the typical cylinder wall (which can be in the form of a conventional probe profile) will start to oscillate when the “shed frequency” equals the natural frequency of the cylinder. Also, oscillations tend to continue at velocities beyond the one causing agreement of frequencies up to a maximum of twice the initiating velocity. If such a condition occurs, the smooth tube probe will oscillate and is liable to snap off where the tube is welded into the fitting.
Accordingly, there exists a long felt, but unresolved need in the industry for a sample probe which resists uncontrolled resonant vibrations and associated destructive forces inherent therewith due to contact with the flow of the fluid process stream, said sample probe using an effective and reliable means to control said forces, while not necessarily being subject to the traditional length and structural parameters as set forth in the above industry standards.